Proceedings of the 4th International Conference on Nanostructures (ICNS4) 12-14 March, 2012, Kish Island, I.R. Iran Synthesis of a New NASICON Structure by Pechini-Type Sol-Gel Method F. Ejehia*, S. P. H. Marashia, D. Haghshenasa, M. Ghaanib, A. Nekahia a Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, 15875-4413, Iran b Department of Material Science, University of Milano Bicocca, Milano, 20125, Italy *[email protected] Abstract: In this research, a new NASICON-type (Na-Super Ionic Conductor) nanostructure, ZrNb(PO4)3 was synthesized through Pechini-type sol-gel method for the first time. The effect of different heat treatment procedures was studied on the purity of the produced structure. X-Ray diffraction patterns (XRD) were obtained to analyze the developed phases and simultaneous thermal analysis (STA) was performed to investigate the phase evolution of the product during heat treatment and to identify the appropriate heating temperatures. The procedure of synthesis was optimized to obtain the maximum purity and consequently the maximum ionic conduction in the final product. The obtained structure can be used as separators, ion-exchange membranes, and sensors. Keywords: NASICON; Zirconium; Pechini method; heat treatment. Introduction + NASICON (Na Super Ionic CONductor) structure is a kind of solid electrolyte which shows high ionic conductivity. In NASICON type structure, with general formula of AM(PO4)3, the mobile ions have enough space to migrate through the surrounding anions. Rhombohedral NASICON structure consists of a threedimensional rigid framework with MO6 octahedra and PO4 tetrahedra sharing common corners . The interstitial spaces among these sites provide appropriate channels for migration of the mobile ions. The choice of an appropriate atom (M) in NASICON structures significantly improves the conduction of mobile ion, especially at higher valences. In the case of tetravalent Zr4+ ion, M should be a penta- (or more) valent ion to make a higher electrostatic interaction with anions and to enhance Zr4+ ion mobility, and accordingly, Nb5+ and Ta5+ can be suitable choices. ZrNb(PO4)3  and ZrTa(PO4)3  structures were first synthesized via solid state method. Among different types of sol-gel routes, Pechini has been favorably employed due to the very stable citrate/metal complexes formed during this process . Moreover, there is no precipitate in the sol which is a very common problem in other methods . In the present work, the effect of single-pass and multiple-pass heat treatment on the synthesis of ZrNb(PO4)3 NASICON structure based on Pechini sol-gel method was studied. Experimental In order to synthesize ZrNb(PO4)3 structure the following precursors with analytical grade were used; ZrOCl2.8H2O (Sigma-Aldrich), NbCl5 (Alfa Aesar), NH4H2PO4 (Sigma- 504 Aldrich), citric acid (Merck), ethylene glycol (Merck), ammonia (Merck), and hydrogen peroxide (Merck). Niobium solution was prepared by dissolving desired amount of NbCl5 in diluted ammonia; thereafter, citric acid (CA) and hydrogen peroxide were added and the mixture was stirred and heated at about 70°C for an hour. The obtained transparent solution was mixed with the water solution of NH4H2PO4. Then, the solution of ZrOCl2.8H2O (containing CA) and ethylene glycol (EG) was added to the sol. The final solution was heated to obtain a gel at 96°C and dried at 150°C in oven for 5 hours. Then, the dry gel was heated at 700°C, 1000°C, and 1200°C as single-pass heat treatments. The structure of the obtained white powders was studied by X-ray powder diffraction (XRD) using CuKα radiation (Bruker D8). The 2θ range in the XRD analysis was between 5 and 70̊ with a step size of 0.031̊. Simultaneous thermal analysis (STA) including thermogravimetric and differential thermal analyses (TG-DTA) was performed on dry gel with a NETZSCH STA 409 PC/PG instrument to improve the heat treatment based on the obtained results. Therefore, the multiple-pass heat treatment was applied at 300°C, 950°C, and 1300°C for 12 hours at each temperature on the dry gel in order to obtain an appropriate structure. Results and Discussion The dry gel contains the main ions in NASICON structure (i.e. zirconium, niobium, and phosphate) and a lot of additional materials which must be eliminated. Therefore, there are two purposes in applying heat treatment: 1) elimination of additional material, 2) crystallization of NASICON structure. In this regard, two types of single-pass and multiple-pass heat treatment Proceedings of the 4th International Conference on Nanostructures (ICNS4) 12-14 March, 2012, Kish Island, I.R. Iran were employed. The single-pass procedure was applied at 3 different temperatures of 700°C (Fig 1.), 1000°C (Fig. 2), and 1200°C (Fig. 3). The XRD pattern of 700°C is revealed an amorphous structure which means that more heating is required. According to Fig. 2, a crystalline phase was produced by heating at 1000°C, however, it needed heating at higher temperatures. As can be seen in Fig. 3, the obtained structure after heating at 1200°C is completely crystalline and the main peaks of NASICON structure are observable even though not very clear. Additionally, there are some impurity phase peaks which probably belong to niobium and zirconium phosphates. Fig. 1. The XRD pattern of the gel dried at 96°C and heated at 700°C. To obtain a better structure, simultaneous thermal analysis (STA) was carried out on dried gels (Fig. 4). The first weight loss at 25-190°C range observed on TG diagram is due to the water elimination of dry gel . The second weight loss at 190-930°C is related to the decomposition of polymer to CO2 and H2O . The third decrease in weight of the sample heat treated at 9301020°C is attributed to the decomposition of starting materials via organic precursor chemical reactions. The last small weight loss at 1020-1370°C in the TG curve is ascribed to the elimination of oxidized carbon from the decomposed organic compounds. DTA curve confirms the above mentioned interpretation. The first endothermic peak at 307°C is due to the decomposition of the polymer to CO2 and H2O . The second peak at 993°C is related to the decomposition of the starting materials with organic precursor chemical reactions between the precursors, which resulted in the formation of a crystalline phase . The third peak at 1344°C shows the sintering temperature which is not accompanied by a noticeable weight loss in TG curve. TG DTA Fig. 4. The STA curves of the dried gel at 96°C. Fig. 2. The XRD pattern of the gel dried at 96°C and heated at 1000°C. Based on the above mentioned temperatures at which considerable weight loss happened, multiple-pass heat treatment was chosen at 300°C, 950°C, and 1300°C for 12 hours to give enough time to eliminate carbon and complete the crystallization. Fig. 5. Shows the XRD pattern of the powder heated at 300°C and 950°C. The crystalline structure indicates that heating in two stages decreases the temperature at which the amorphous phase disapears. Fig. 3. The XRD pattern of the gel dried at 96°C and heated at 1200°C for 12 hours. The impurity peaks are marked. 505 Proceedings of the 4th International Conference on Nanostructures (ICNS4) 12-14 March, 2012, Kish Island, I.R. Iran influence of the heat treatment on the purity of the product and the discussed evolutions during the heating can help to improve the procedures of synthesis. Increasing the purity of structure would result in enhancement of ionic conduction and improvement of the performance of ZrNb(PO4)3 structure. Fig. 5. The XRD pattern of the gel dried at 96°C and heated at 300°C, 950°C for 12 hours. The result related to the final product implies that a pure NASICON phase with sharp and distinct peaks (Fig. 6) is formed which perfectly matches the previous reported work . Fig. 6. The XRD pattern of the gel dried at 96°C and heated at 300°C, 950°C, and 1300°C for 12 hours. By calculating the lattice parameters of pure powder for hexagonal system, the values of “a” and “c” were obtained as 0.89 nm and 2.24 nm, respectively. The crystallite size of this sample has a mean value of 60 nm. Conclusions ZrNb(PO4)3 NASICON-type structure was synthesized by modified Pechini type method for the first time. The method was used to obtain more homogeneity and purity in the final solid electrolyte. The results confirm the 506 References  H. Y. P. 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